432 new feature willard chandler density parameter

[Marcello-Sega]

The Willard Chandler method takes a parameter c that defines the density to use to describe the isosurface. In Pytim, this defaults to the average of the minimum and maximum density. However, sometimes one might want to use a different value, so I've just added an argument to the WillardChandler class so we can!

I also deleted .travis.yml., which you can undo if you'd like. I wasn't able to clone the repository on Windows because the file name is invalid. I suspect it should be .travis.yml (without the trailing .), so it probably needs renaming anyway.

Please make sure that your pull request satisfies the following requirements:

If you have written a new method in one of the available modules

• [x] Document it with a docstring
• [ ] Add a short, deterministic test in the docstring
• [x] Explain briefly what the patch is about in the pull request

Please let me know if you want me to add another test to the docstring; it might make it a bit lengthy.

[Marcello-Sega]

@a-ws-m could you do your changes on this PR? If you don't have time let me know, I can take care. Thanks!

[a-ws-m]

@Marcello-Sega I'm away for a week but I can do them when I'm back :)

[Marcello-Sega]

No worries, then I'll take care of it.

[Marcello-Sega]

I've decided to add a a surface layer identification mechanism for the Willard Chandler method, I will include the docstring test in that pull request linked to #434